Ecology

Cole, C. Vernon; Innis, George S.; Stewart, J. W. B.

doi: 10.2307/1935104pmid: N/A

A simulation model of the phosphorus cycle in semiarid grasslands was developed and tested. When used with data sets for biotic and abiotic driving variables of the Pawnee (Colorado) and Matador (Saskatchewan) Sites, this model predicted plant and decomposer uptakes and turnover rates of the principal phosphorus compartments. Daily P requirements for plant and decomposer uptake are taken from pools of labile inorganic P in each soil layer. These pools are replenished mainly by mineralization of labile organic P as well as leaching of water—soluble forms from standing dead biomass and litter. Phosphorus solubility (recalculated daily from relationships to the labile inorganic pools), soil H2O content, and rates of diffusion of P through soil are the major controls on rates of uptake by the active fraction of the live root biomass. Model operation was found to be more sensitive to soil parameters than to plant parameters. Simulation results indicated rates of decomposer uptake 4—4x greater than plant uptake in semiarid grasslands. Simulated P concentrations in live plant tops were highly responsive to the pattern of seasonal rainfall which agreed well with published data. The most critical informational needs revealed by model development and operation were in the areas of activity and morphology of roots and the rates of mineralization of organic P as affected by soil depth.

Stark, Nellie M.

doi: 10.2307/1935105pmid: N/A

Twenty control burns performed with a wide range of fuel loadings and moisture conditions were used to study the effectiveness of old fuel reduction under standing Douglas—fir/larch forest. This paper reports the influence of burning on nutrient retention and loss from the soil. Sixty % of the fires were successful in reducing residual fuels with no accelerated loss of nutrients below the root zone. Net less of Ca+2 and Mg+2 occurred below the root zone when soil surface temperatures exceeded 300°C, but were insignificant when soil surface temperatures remained below 200—300°C. No other elements were loss (net) from the soil as a result of burning. Precipitation on control soils delivers as much Ca+2 as is normally lost below the root zone in the absence of fire. Iron concentration in the soil water is a good indicator of the intensity of burn. The hotter the fire, the less iron in the soil water as a result of the alkaline pH. Ash shows a definite pattern of nutrient release under the influence of precipitation. Homogeneous subsamples of litter showed predictable nutrients losses when ignited at different temperatures. Overland flow and surface erosion are of little significance on this soil type. Decomposition of Douglas—fir litter was only slightly more rapid on hot burned substrates than on control (unburned) substrates. When the biological life concept was applied to this soil, it showed that this soil is young and capable of withstanding many years of cyclic intensive burns.

Randolph, Polley Ann; Randolph, J. C.; Mattingly, Kevin; Foster, Margaret Mead

doi: 10.2307/1935106pmid: N/A

Individual organism costs of reproduction were investigated by a series of observations and experiments with the cotton rat, Sigmodon hispidus. Energy budget techniques, including respirometry, showed that ingestion in pregnant rats was increased 25%, while that in lactating rats was increased 66%, over nonreproductive rats. Digestive efficiency of rats fed a commercial standard diet was 84% and did not change as a result of pregnancy or lactation. Respiration per gram of reproductive rats was not significantly different from nonreproductive rats. Twenty—six days of pregnancy required an average additional ingestion of 221.8 kcal (=928.0 kJ); 12 days of lactation required an extra 270.7 kcal(=1,132.6 kJ). Total cost per offspring 12 days of age was 100.5 kcal(=420.5 kJ). The total cost of producing a litter was a linear function of litter size. Two—thirds of the energy accumulated by a pregnant rat was not in the neonates but deposited as storage and mobilized during lactation when the precocial young grew faster than the maternal assimilation alone could support. This use of stored energy increased the apparent efficiency of lactation above that found in other wild rodents. Neonatal litters of cotton rats contained two to seven young (Z = 5). The sizes of these individuals were not affected by litter size although their subsequent early growth rates were. Individuals from larger litters grew more slowly.

Eggers, Douglas M.

doi: 10.2307/1935107pmid: N/A

The nature of prey selection by planktivorous fish is presented as a model. The model conceptualization is based on the predation cycle, and the effects of prey distribution, encounter rate, handling time, capture success, and optimal foraging on food selection are explicit. The model as presented assumes a multiple species prey assemblage admitting within—species size variation. Conditions are given so that the optimal strategy of prey utilization can be expressed as a breadth of diet, and the specific components of the predator—prey interaction of fish and zoo plankton are discussed in detail. A reduced form of model is applied to two experimental situations. Specific factors which influence prey selection are also discussed.

Schaffer, William M.; Rosenzweig, Michael L.

doi: 10.2307/1935108pmid: N/A

In a recent theoretical study of life history evolution, it was suggested that, for some species, there might exist more than one set of age—specific reproductive expenditures corresponding to local maxima in fitness. The present paper points out that the argument supporting this conclusion was in error, but that the conclusion itself is correct. In addition, the suggestion that the same adaptive landscape can obtain local optima corresponding to both semelparity and iteroparity is confirmed. The circumstances under which variation in reproductive efforts should be canalized are also discussed.

Foster, Mercedes S.

doi: 10.2307/1935109pmid: N/A

The Long—tailed Manakin (Chiroxiphia linearis) was studied from 1971 to 1974 in Guanacaste Province, Costa Rico. From late April until early June 1973 the normal food of this species, i.e., ripe fruits of Ardisia revoluta Humboldt, Bonpland, and Kunth (Myrsinaceae) and Stemmadenia donnell—smithii (Rose) Woodson (Apocynaceae), was unavailable. The birds switched to a diet of green Ardisia fruits upon which they normally do not feed. The primary components of the normal and alternate diets (fruits and varying ripeness) were analyzed to determine differences in nutrient and energy contents in an attempt to quantify the impact of the food shortage. Birds needed to eat 6.6x as many green fruits of A. revoluta to obtain the same metabolizable energy contained in a ripe fruit and 4.5, 8.5, and 18.6x as many to obtain comparable amounts of H2), protein, and trichloroacetic acid—soluble carbohydrates, respectively. The birds had to eat 1.4, 8.3, 4.8, and 5.8 x as many green A. revoluta fruits to obtain the same amounts of water, lipids, protein, and metabolizable energy contained in one feeding unit of ripe S. donnell—smithii. Feeding on green fruit probably is the best way for Long—tailed Manakin to cope with a period of food scarcity for several reasons. The fact that the birds maintained their normal weights during this period supports this idea. Experiments showed that seeds from green and ripe fruits germinated with almost equal frequency. Maturation appears to influence primarily the speed of germination. It also enhances dispersal by making the fruits more attractive to dispersal agents.

Bertness, Mark D.

doi: 10.2307/1935110pmid: N/A

Four intertidal carnivorous gastropods in the Puget Sound region. Thais lamellosa, Thais emarginata, Thais canaliculata, and Searlesia dira, exhibit intraspecific shore—level size gradients, with shell size increasing with decreasing tidal height, on a majority of the 13 beaches examined. Thais lamellosa and T. emarginata respond by photo—orthokinesis and geotaxis to gradients of light and beach elevation to establish and maintain their nonrandom size—class distributions. A previously published hypothesis that size gradients place the prereproductive members of a population in the least stressful portion of a species' intertidal range is examined. Evidence suggests that thaid size gradients are in response to similar size gradients in their major food source, Balanus spp. This promotes an energetically efficient predator—prey size selection relationship and facilitates intra and interspecific resource partitioning.

Beiswenger, Ronald E.

doi: 10.2307/1935111pmid: N/A

Tadpoles of the American toad, Bufo americanus, undergo a diel cycle of distribution and activity. Tadpoles spend the night scattered throughout all but the shallowest parts of ponds, where it is cooler than in nearby deeper areas. They begin swimming and feeding more frequently as light intensity increases in the morning, and when shallow areas begin to warm, they move into them. Most are in aggregations in the upper 1—2° of temperature gradients through midday. During late afternoon, they move from shallow water and by evening are scattered and inactive. This diel cycle is closely correlated with changes in light. Light triggers activity in the morning, and swimming and feeding are depressed on cloudy days. Diminishing light causes individuals to disperse from aggregations and become inactive. Once active, tadpoles are attracted to particular microhabitats by heat and light. Temperature gradients are primarily responsible for bringing tadpoles together in sufficient density for aggregations to develop, but thermal effects do not act along in controlling aggregations.

Boddington, M. J.; Mettrick, D. F.

doi: 10.2307/1935112pmid: N/A

Populations of the flatworm Dugesia polychroa were studied in the laboratory over 42 mo under constant regimes of temperature, food, and space. Three populations completed four reproductive cycles, a fourth achieved an equilibrium size in its third cycle, while a fifth became extinct after its second reproductive period. The populations fluctuated in numbers and biomass during each cycle. As the populations matured, the mean population size (in terms of numbers and biomass) increased, as did the mean number of adults present. There were also increases in the length of the cycles, the reproductive periods, and total number of cocoons laid; there were decreases in the rate of cocoon production, adult fecundity, viability of cocoons, amplitude of the fluctuations in, and the average of the biomass turnover rate. In general, mortality balanced recruitment with the majority of mortality occurring at the time of recruitment. Only the absolute amount of food presented was found to be of importance in influencing the above parameters. The relationship between food availability and average biomass standing stock gave a value of 0.76, which was used to obtain good comparative abundance estimates for a field situation. Modifications of the above parameters were what might be expected from a density dependent organism and the results support field observations on triclads in general, and on D. polychroa in particular, showing that their populations are regulated by intraspecific competition for food.

Huey, Raymond B.; Pianka, Eric R.

doi: 10.2307/1935113pmid: N/A

Two semiarboreal skinks (Mabuya) are narrowly sympatric in the Kalahari desert, whereas two terrestrial species are broadly sympatric both with one another and with the semiarboreal species. We attempt to discern reasons for these differing distributional patterns. The narrow zone of sympatry of the semiarboreal species is partially congruent with zones of habitat change (rainfall and vegetation), suggesting that the narrow zone probably reflects adaptations of these species of geographic discontinuities of the physical environment. However, the near identity of niches (position on sandridges, microhabitats, body temperatures, times of activity, as well as types and sizes of prey) of the semiarboreal species relative to other species pairs also suggest that intense competition could restrict the zone of sympatry.